DE102017112256A1 - METHOD FOR DETECTING A CONTACT ERROR IN A PHOTOVOLTAIC SYSTEM - Google Patents

Abstract

What is described is a method for detecting a contact fault in a photovoltaic system (10) which comprises an inverter (13) and a photovoltaic generator (11) connected via DC lines (12a, 12b) to the inverter (13), wherein the inverter (13) is coupling means (15) for coupling a first alternating voltage signal with communication frequencies (31) in a first frequency range between 125 and 150 kHz in the DC lines (12a, 12b) and communicates by means of the first AC voltage signal to the photovoltaic generator (11). On the photovoltaic generator (11) a decoupling means (16) for decoupling the first alternating voltage signal and between the inverter (13) and the photovoltaic generator (11) decoupling means (19) for decoupling the impedance of the photovoltaic generator (11) is arranged, so that the photovoltaic generator (11) is decoupled from a transmission path between the inverter (11) and decoupling means (19). AC currents in and AC voltages between the DC lines (12a, 12b) are measured to determine an impedance characteristic. The method is characterized in that a second alternating voltage signal having measuring frequencies (32) is generated by means of the coupling-in means, whereby a frequency-dependent impedance characteristic (30) in a second frequency range between 50 kHz and 350 kHz, preferably between 75 kHz and 200 kHz is determined, wherein the second frequency range includes the first frequency range, and that a contact error in the photovoltaic system (10) by means of an analysis of the frequency-dependent impedance curve (30) is detected. The method can be carried out in an inverter (13). A photovoltaic system (10) may have such an inverter (13).

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for operating a photovoltaic system, which is set up for supplying electrical power to an AC voltage network, an inverter for a photovoltaic system, as well as such a photovoltaic system.

STATE OF THE ART

A contact fault in a DC circuit of a photovoltaic system can cause significant heating of the environment of the fault and lead to arcing. In addition to a malfunction, in particular an interruption of the DC circuit, thereby high damage can be caused, and in extreme cases may cause a fire of the photovoltaic system. To minimize this risk, various methods for the early detection of a contact error are known.

Basically, a contact fault is characterized by a change in the series resistance of the affected component. Therefore, many of the known methods for contact fault detection are based on modeling and analyzing the impedance of the DC circuit of the photovoltaic system. In addition, the impedances of some of these components are temperature-dependent, and the impedance of the photovoltaic generators is subject to aging effects, in particular the so-called potential-induced degradation, by means of the DC impedance an impedance change can be caused which is very similar to a change in impedance caused by a contact error. Another disadvantage of the known method is the influence of the impedance analysis by the operation of the photovoltaic system, so that the known methods can be performed only in times without solar irradiation on the photovoltaic generators, in particular only at night. It should be noted that no power from the photovoltaic generators is available in these times, so that an additional energy source for performing the known method for contact fault detection must be provided.

From the WO2012000533A1 a method for monitoring a photovoltaic system is known, in which by means of a signal input means a test signal is fed into a photovoltaic generator and coupled by means of a signal extraction means a response signal, wherein in the case of several parallel connected photovoltaic generators each a pair of signal coupling means and signal extraction means for coupling and decoupling each one Test signal per photovoltaic generator is provided.

From the EP2388602A1 a method for the diagnosis of contacts of a photovoltaic system is known, in which a multiple frequencies comprehensive test signal coupled into DC lines, coupled out a response signal from the photovoltaic system and a generator impedance is determined by means of an evaluation of the response signal. In order to monitor the contacts, the alternating current behavior of the photovoltaic system is modeled on the basis of an amount and a phase information of the determined generator impedance, and from the model a series impedance is determined which contains information about the state of the contacts. A comparable procedure is also from the EP2388603A1 known.

OBJECT OF THE INVENTION

The invention has for its object to provide a method for detecting a contact failure in a photovoltaic system, an inverter for a photovoltaic system and a photovoltaic system equipped with it, so that improved fire safety and monitoring of photovoltaic systems simpler and more robust than with known from the prior art Procedure can be performed.

SOLUTION

The object is achieved by a method for detecting a contact fault in a photovoltaic system with the features of patent claim 1, an inverter according to claim 5, and by a photovoltaic system with the features of claim 7. Preferred embodiments are defined in the dependent claims.

DESCRIPTION OF THE INVENTION

In a method according to the invention for detecting a contact fault in a photovoltaic system, the photovoltaic system comprises an inverter and a photovoltaic generator connected via DC lines to the inverter. The inverter includes coupling means for coupling a first AC signal having communication frequencies in a first frequency range between 125 and 150 kHz into the DC lines. By means of the first alternating voltage signal, the inverter communicates with the photovoltaic generator, wherein at the photovoltaic generator a decoupling means for Decoupling of the first alternating voltage signal is arranged, so that the photovoltaic generator is decoupled from a transmission path between the inverter and the decoupling means. Furthermore, a decoupling means for decoupling the impedance of the photovoltaic generator is arranged between the inverter and the photovoltaic generator, and alternating currents are measured in and AC voltages between the DC lines.

By means of the coupling means, a second alternating voltage signal is generated with measuring frequencies, wherein by means of the second alternating voltage signal and the measured alternating currents, a frequency-dependent impedance characteristic in a second frequency range between 50 kHz and 350 kHz, preferably between 75 kHz and 200 kHz is determined, and wherein the second frequency range the includes first frequency range. Finally, a contact error in the photovoltaic system is detected by means of an analysis of the impedance curve.

By introducing the decoupling means between the inverter and the photovoltaic generator, as well as due to resonant properties of the decoupling results in the first frequency range between 125 and 150 kHz, a defined course of the impedance of the transmission path between the coupling means and the decoupling means. This defined course of the impedance is used to provide a stable communication channel between the inverter and the photovoltaic generator.

The invention is based on the recognition that the defined course of the impedance offers considerable advantages, which go beyond the provision of the stable communication channel. In particular, the first and the second frequency range are well below frequency ranges in which the DC lines between the inverter and photovoltaic generator usually cause resonance effects; the latter are usually well above 300 kHz. In addition, the decoupling acts as a bypass for the AC signals so that they do not reach the photovoltaic generator and any resonance effects of the photovoltaic generator have no effect on the transmission path between coupling and decoupling. The impedance of the photovoltaic generator is effectively shielded by the decoupling of the transmission path between the inverter and decoupling.

Thus, the course of the impedance in the second frequency range between 50 kHz and 350 kHz and in particular between 75 kHz and 200 kHz almost exclusively determined by the line impedances of the DC lines and any series impedances of contact points in the distance from the inverter via the DC lines to the decoupling. This course of the impedance in the second frequency range is largely constant, in particular in the absence of contact points, and in particular is not influenced by temperature, radiation or load-dependent impedances of the photovoltaic generator or resonance points of the DC lines.

The inventive method can be particularly advantageous combined with a known method for impedance spectroscopy, as for example from the EP2388602A1 is known. The course of the impedance determined from the measured alternating currents and alternating voltages is optionally analyzed, taking into account the DC voltage at the input of the inverter, for example with the aid of a model approach determined in advance or situationally, and the series impedance of the DC cables including the contact points, in particular those at the terminals, is calculated therefrom of the inverter and decoupling determined. This series impedance contains all the information necessary to detect a contact fault.

An inverter according to the invention comprises a controller which is set up to carry out the method according to the invention. For this purpose, the inverter comprises a coupling means for coupling a first alternating voltage signal with communication frequencies in a first frequency range between 125 and 150 kHz in the DC lines, so that the inverter is able to communicate with the photovoltaic generator by means of the first AC voltage signal. Furthermore, the inverter comprises current and voltage sensors for measuring a current in or between DC lines which are connected to DC terminals of the inverter.

In one embodiment of the invention, the inverter comprises connections for connecting a plurality of photovoltaic generators, wherein the photovoltaic generators are connected in parallel within the inverter or in a connection unit belonging to the inverter. In this case, each photovoltaic generator is assigned a respective current measuring means within the inverter or the connection unit. However, the coupling means is arranged centrally within the inverter or the connection unit such that the first and the second alternating voltage signal is simultaneously coupled into the direct current lines leading to the parallel-connected photovoltaic generators during operation of the inverter. This makes it particularly easy to detect contact errors for a number separately connected photovoltaic generators without having to provide for each of the photovoltaic generators own launching means, as is necessary in conventional inverters and corresponding methods.

A photovoltaic system according to the invention comprises a photovoltaic generator, a decoupling means for decoupling the first AC voltage signal, an inverter having the above features according to the invention and a decoupling means for decoupling the impedance of the photovoltaic generator from a transmission path between the inverter and the decoupling means.

In one embodiment of the photovoltaic system, the decoupling means may be designed as a parallel resonant circuit or as a series resonant circuit. The decoupling means may comprise a capacitance which is arranged in parallel with the photovoltaic generator between the DC lines. Alternatively or additionally, the decoupling means may comprise a bandstop filter which is arranged in one of the DC lines.

list of figures

• 1 zeigt eine Photovoltaikanlage mit einem Wechselrichter, einem Photovoltaikgenerator und Vorrichtungen zur Durchführung des erfindungsgemäßen Verfahrens,
• 2 zeigt ein Beispiel eines frequenzabhängigen Impedanzverlaufs einer Übertragungsstrecke zwischen dem Wechselrichter und dem Photovoltaikgenerator,
• 3 zeigt eine Ausführungsform des erfindungsgemäßen Verfahrens in Form eines Flussdiagramms, und
• 4 zeigt eine weitere Ausführungsform einer Photovoltaikanlage.

In the following the invention will be further explained and described with reference to embodiments shown in the figures.

• 1 shows a photovoltaic system with an inverter, a photovoltaic generator and devices for carrying out the method according to the invention,
• 2 shows an example of a frequency-dependent impedance curve of a transmission path between the inverter and the photovoltaic generator,
• 3 shows an embodiment of the method according to the invention in the form of a flow chart, and
• 4 shows a further embodiment of a photovoltaic system.

DESCRIPTION OF THE FIGURES

1 shows a photovoltaic system 10 with a photovoltaic generator (PV generator) 11 which may consist of one or more PV modules connected in parallel and / or in series and via DC cables 12a . 12b to an inverter 13 connected. The inverter 13 converts it from the PV generator 11 converts direct current into alternating current and feeds it into an alternating voltage network 14 one.

The inverter 13 includes a coupling agent 15 for injecting an AC signal into one of the DC lines 12a . 12b , The coupling agent 15 can be implemented in various forms, for example as in 1 represented with an AC voltage source 15a connected to a primary side of a transformer 15b is connected, the secondary side of the transformer 15b such in the DC line 12a is integrated into the DC line 12a an AC signal is injected. Further embodiments of the coupling-in means 15 are known in the art.

The thus coupled AC signal is transmitted through the DC lines 12a . 12b to the PV generator 11 transfer. Near the PV generator 11 , For example, in a junction box of a photovoltaic module of the PV generator 11 , is a decoupling agent 16 arranged. The decoupling agent 16 can be implemented in various forms, for example as in 1 shown as RLC parallel resonant circuit with one in the DC line 12a arranged inductance 16a as well as a capacity 16b and a resistance 16c , each electrically parallel to the inductance 16a are arranged; Alternatively, other embodiments for the decoupling means 16 conceivable, for example, a series resonant circuit. By means of a voltage measurement, not shown, that can be the PV generator 11 transmitted AC voltage signal at the decoupling means 16 be tapped, for example, as a voltage drop across the resistor 16c or parallel to the decoupling agent 16 and a decoupling agent 19 ,

The frequency response of a transfer function for the AC signal depends directly on the frequency-dependent course of the impedance of the transmission path between the coupling means 15 and the decoupling means 16 from. The impedance of the transmission path is composed of several shares, in particular line impedances L _LTG the DC lines 12a . 12b , any series impedances of contact points, as well as in conventional photovoltaic systems, the impedance of the PV generator 11 include. The decoupling agent 19 that between decoupling agent 16 and PV generator 11 is arranged, serves to decouple the complex and also time-varying impedance of the PV generator 11 , so that the impedance of the PV generator 11 from the transmission path between Einkoppelmittel 15 and decoupling agent 16 is decoupled. This results in a defined impedance curve between coupling-in means 15 and decoupling agent 16 generated, wherein the impedance profile is largely constant in time, in particular in the absence of contact errors. Such a transmission path with a defined impedance curve, in particular in the range of selected frequencies, is particularly good for communication between the coupling-in means 15 and the decoupling means 16 suitable. As a result, a defined and limited reception level range can be calculated and indicated, and the ratio of minimum reception level to maximum reception level becomes smaller.

That of the coupling agent 15 over the DC lines 12a . 12b to the decoupling agent 16 transmitted AC signal can be used for unidirectional communication with another, at the PV generator 11 arranged electrical or electronic circuit can be used (in 1 not shown). For example, a circuit breaker or a short-circuit switch on the PV generator 11 be arranged, the switching state by means of one of the coupling means 15 in the DC lines 12a . 12b coupled and optionally coded AC signal is controlled.

The inverter 13 further comprises a current measuring means 17 and a voltage measuring means 18 , The current measurement 17 is in one of the DC power lines 12a . 12b arranged and for measuring the in the DC lines 12a . 12b flowing stream furnished. This stream can be next to that from the PV generator 11 produced direct current also have AC components that due to the of the coupling means 15 coupled AC signal in the DC lines 12a . 12b be induced. Alternatively, the inverter 13 also several current measuring means 17 For example, a current measuring means optimized for the measurement of direct currents 17 and at least one further current measuring means optimized for the measurement of alternating currents in a frequency range of the second alternating voltage signal 17 ,

The voltage measuring device 18 is between the DC lines 12a . 12b arranged and for measuring the potential difference between the DC lines 12a and 12b set up. As with the current measurement 17 can the inverter 13 also several voltage measuring devices 18 each optimized for measuring DC voltages or AC voltages in one or more frequency ranges.

At the PV generator 11 is a decoupling agent 19 arranged. The decoupling agent 19 offers the from the coupling agent 15 in the DC power lines 12a . 12b generated alternating currents bypass the PV generator 11 so these AC currents do not have the PV generator 11 have to flow and the PV generator 11 thus with respect to AC voltages and AC currents in the DC lines 12a . 12b is decoupled.

2 shows an exemplary impedance curve 30 the amount of impedance | Z | the transmission path consisting of DC cables 12a . 12b , Decoupling agent 16 and decoupling agent 19 , as well as adjacent contact points. Due to the decoupling agent 19 is the impedance curve 30 the impedance | Z | not by the impedance of the PV generator 11 affected.

In the upper part of the 2 is the impedance curve 30 the amount of impedance | Z | represented over a frequency range of about 50 kHz to 450 kHz. A first AC voltage signal for communication between the coupling means 15 and the decoupling means 16 is provided, by the coupling means 15 in the DC lines 12a . 12b be fed and selected communication frequencies 31 here in the range of about 131 kHz and 146 kHz and in 2 are shown as vertical lines.

In the lower part of the 2 is an enlarged section of the impedance curve 30 the amount of impedance | Z | represented over a frequency range of about 75 kHz to about 200 kHz. To determine the impedance curve 30 In this frequency range, a second alternating voltage signal can be used, for example by the coupling means 15 in the DC lines 12a . 12b is fed. The second alternating voltage signal can have different, in particular discrete, measurement frequencies 32 have, wherein the second alternating voltage signal in particular at any time a single of the measurement frequencies 32 to the amount of impedance | Z | to be determined at this single measuring frequency. In principle, however, the measurement signal can also have several or all measurement frequencies 32 at the same time. The impedance curve 30 then results from multiple determination of the impedances | Z | at different measuring frequencies 32 , where the measurement frequencies 32 extend over a frequency range that covers the communication frequencies 31 includes and in particular substantially symmetrical about the communication frequencies 31 extends.

By the arrangement of the decoupling agent 16 and the decoupling agent 19 at the PV generator 11 indicates the impedance curve 30 especially in the field of communication frequencies 31 a defined course, in particular neither by the operational impedance of the PV generator 11 nor by any higher-frequency resonances of the DC lines 12a . 12b is affected. On the one hand, this is necessary to ensure stable communication between the coupling means 15 and the decoupling means 16 to ensure. On the other hand, this property can be advantageously exploited by any contact errors a change in the series impedance of the DC cables 12a . 12b cause, which in turn directly affect the impedance curve 30 affect and insofar as a change in the impedance curve 30 can be detected. The corresponding procedure is below in connection with 3 explained.

3 shows a flowchart of an embodiment of the method according to the invention. The procedure starts with step S1 in which, for example, the photovoltaic system 10 in total goes into operation. In step S2 is a first AC signal (first AC signal) in a first frequency range from the coupling means 15 in the DC lines 12a . 12b imprinted. In step S3 is the first AC signal from the decoupling 16 decoupled and from an electrical or electronic circuit on the PV generator 11 exploited such that an open circuit and / or short-circuit switch arranged on the PV generator is switched to a state which is a transmission of electrical power from the PV generator 11 to the inverter 13 allows. In step S4 then can the electric power of the PV generator 11 in particular by setting the DC voltage (DC voltage) at the input of the inverter 13 be optimized so that, for example, a point of maximum power on a current-voltage characteristic of the PV generator 11 is set.

In step S5 becomes a second AC signal (second AC signal) in a second frequency range from the coupling means 15 in the DC lines 12a . 12b impressed, wherein the frequency of the second AC signal is varied over the second frequency range and comprises the first frequency range. In step S6 becomes the course of the alternating current (AC current) induced in the DC lines due to the second AC signal 12a . 12b measured. In step S7 This alternating current curve takes into account the DC voltage also measured at the input of the inverter 13 analyzed by means of a model approach determined in advance or situationally and from the impedance curve 30 the series impedance Z _{L of} the DC cables 12a . 12b determined. Finally, in step S8 the determined series impedance Z _{L of} the DC cables 12a . 12b is compared with a reference series impedance Z _{L, Ref} , which was determined, for example, in a previous run of the method or purely analytically, with this previous run or the analytical approach of faultless DC lines 12a . 12b is assumed without presence of contact errors. Alternatively or additionally, in step S8 the one in step S7 determined impedance curve 30 be subjected to a pattern recognition to detect a contact error, which has a unique pattern to the impedance curve 30 added. When in step S8 a significant change in the series impedance is detected or a pattern generated by a contact error is detected, is in step S9 indicates the presence of a contact fault in a suitable form, for example to an operator of the photovoltaic system, and the photovoltaic system 10 optionally switched off, in particular by an interruption of the direct current in the DC lines 12a . 12b , When in step S8 no significant change in the series impedance is detected or no pattern generated by a contact fault is detected, the photovoltaic system 10 continue to operate and the process jumps back to step S5 in order to update the determined series impedance Z _L in a further run of the method, which is carried out directly afterwards or also some time later, for example after a few minutes or hours.

4 shows a further embodiment of a photovoltaic system 10 with an inverter 40 , the opposite of the inverter 13 according to 1 has further DC inputs, so that in addition to the PV generator 11 another PV generator 41 over other DC lines 12c . 12d to the inverter 40 can be connected. The further PV generator 41 is inside the inverter 40 parallel to the PV generator 11 connected; Alternatively, this parallel connection can also be outside the inverter 40 take place, for example, in an additional connection unit, not shown here for merging multiple PV generators 11 . 41 ,

For detecting contact faults in the DC cables 12a . 12b . 12c . 12d to the PV generators 11 . 41 it is necessary for every connected PV generator 11 . 41 a corresponding decoupling agent 16 , a decoupling agent 19 , as well as a current measuring agent 17 assigned. The detection of contact errors is then particularly easy, because a total of only one coupling agent 15 and only one voltage measuring means 18 for determining the impedance characteristics of the transmission links between the coupling means 15 and then several PV generators 11 . 41 necessary is.

LIST OF REFERENCE NUMBERS

10

photovoltaic system

11

photovoltaic generator

12a, 12b

DC lines

13

inverter

14

AC network

15

coupling means

15a

AC voltage source

15b

exchangers

16

decoupling

16a

inductance

16b

capacity

16c

resistance

17

Current measuring means

18

Voltage measuring means

19

Entkoppelmittel

30

impedance curve

31

communication frequencies

32

measuring frequencies

40

inverter

41

photovoltaic generator

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2012000533 A1 [0004]
• EP 2388602 A1 [0005, 0013]
• EP 2388603 A1 [0005]

Claims (7)

Method for detecting a contact fault in a photovoltaic system (10), wherein the photovoltaic system (10) comprises an inverter (13) and a photovoltaic generator (11) connected to the inverter (13) via direct current lines (12a, 12b), the inverter (13 ) Coupling means (15) for coupling a first alternating voltage signal with communication frequencies (31) in a first frequency range between 125 and 150 kHz in the DC lines (12a, 12b) and communicates by means of the first AC voltage signal with the photovoltaic generator (11), wherein at the photovoltaic generator (11) a decoupling means (16) for decoupling the first AC voltage signal and between the inverter (13) and the photovoltaic generator (11) a decoupling means (19) for decoupling the impedance of the photovoltaic generator (11) is arranged, so that the photovoltaic generator (11) from a transmission path between inverter (11) and decoupling elmittel (19) is decoupled, and wherein alternating currents in and AC voltages between the DC lines (12a, 12b) are measured, characterized in that by means of the coupling means (15), a second AC voltage signal with measurement frequencies (32) is generated, wherein by means of the second AC voltage signal and the measured alternating currents, a frequency-dependent impedance curve (30) is determined in a second frequency range between 50 kHz and 350 kHz, preferably between 75 kHz and 200 kHz, the second frequency range comprising the first frequency range, and a contact error in the photovoltaic system (10). is detected by means of an analysis of the frequency-dependent impedance curve (30). Inverter (13) for a photovoltaic system (11), wherein the inverter (13) comprises a controller which is adapted to carry out a method according to one of the preceding claims. Inverter (13) to Claim 5 wherein the inverter (13) has terminals for connecting a plurality of photovoltaic generators (11, 41), wherein the photovoltaic generators (11, 41) are connected in parallel within the inverter (13) or in a terminal unit associated with the inverter (13) and each photovoltaic generator each one current measuring means (17) within the inverter (13) or the connection unit is assigned, and wherein the coupling means (15) within the inverter (13) or the connection unit is arranged centrally such that the first and the second AC voltage signal during operation of the inverter (13) at the same time in the leading parallel to the photovoltaic generators (11, 41) leading direct current lines (12a, 12b, 12c, 12d) is coupled .. Photovoltaic system (10) with a photovoltaic generator (11), a decoupling means (16) for decoupling the first AC voltage signal, an inverter according to Claim 5 or 6 and a decoupling means (19) for decoupling the impedance of the photovoltaic generator (11) from a transmission path between the inverter (11) and decoupling means (19). Photovoltaic system (10) after Claim 4 , wherein the decoupling means (16) is designed as a parallel resonant circuit or a series resonant circuit. Photovoltaic system (10) according to one of Claims 4 to 5 wherein the decoupling means (19) comprises a capacitor arranged in parallel with the photovoltaic generator (11) between the dc power lines (12a, 12b). Method according to one of Claims 4 to 6 wherein the decoupling means (19) comprises a band-stop filter arranged in one of the direct-current lines (12a, 12b).

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